The United States yesterday went into official mourning for the loss of the space shuttle Challenger and its crew of seven as the grim search for debris stretched over a 7,200 square-mile area of the Atlantic.

Early speculation on the cause of the accident is focusing on the craft's large external fuel tank carrying some 385,000 gallons of liquid hydrogen and 140,000 gallons of oxygen at takeoff.

But at a news conference from Cape Canaveral last night, Nasa officials refused to give credence to any particular theories and put the stress on the search for debris and the gathering of data from the sophisticated computer tape and telemetry which controlled Challenger's fate.

One danger area which is being openly discussed here was the so-called 'intertank' which separated the liquid oxygen tank from the liquid hydrogen tank. A rupture or leak in the pipes of the intertank could have mixed the two fuels, making them highly volatile and turning Challenger into an enormous bomb. It is possible that the sub-zero temperatures at Cape Canaveral before the launch could have played a part.

It emerged last night that Rockwell International, one of the main shuttle contractors, had raised questions in the hours before the launch about the effect of the low temperatures on safety. Nasa said yesterday that its technicians and outside contractors had discussed this point but decided, on examining the data, that everything was fine. The possibility of a sharp object such as a blade, bolt or other fragment puncturing the external fuel tanks has also been discussed.

Two solid rockets were deliberately destroyed after separating from the fireball left by the Challenger explosion. An air force officer transmitted a radio signal to explosive packages aboard the boosters. A Nasa official said: 'There was an indication that one of the solids was heading for a populated area and he took the correct action.'

The signal to destroy the solid booster rockets came 30 seconds after the Challenger explosion.

The discussion of the causes of the Challenger catastrophe was conducted in the same sober terms as the national remembrance of what had been lost in terms of lives, material and the future of the shuttle programme. President Reagan ordered all flags to be flown at half-mast on government buildings and military outposts around the world as the Nasa board of inquiry and preparations for extensive Senate and House investigatory hearings began.

At a morning church service in Concord, New Hampshire, the home town of the teacher-astronaut Christa McAuliffe, 300 scholchildren described their feelings. The sombre mood was seen as reminiscent of America in the days after the assassinations of John F Kennedy and Martin Luther King in the 60s.

President and Mrs Reagan will lead the memorial service for the seven lost American heroes when they travel to Houston, home of Mission Control, tomorrow. The dead astronauts were a profile of Americans in the mid-1980s: the commander, Francis Scobee, and the pilot, Michael Smith, were Vietnam veterans: Judith Resnik was the single professional woman working as mission specialist: Ellison Onizuka a Japanese-American and mission specialist: Ronald McNair, a black and mission specialist: Gregory Jarvis, of Hughes Aircraft, the aerospace scientist, and Christa McAuliffe, the instant darling of the country's children.

At a Cape Canaveral press conference yesterday, the acting Nasa administrator, Mr William Graham, made it plain that space flight had to go on. 'While we grieve for our lost colleagues and families left behind, the Nasa team is dedicated to understanding very thoroughly what happened yesterday. We're dedicated to pressing on.'

But Mr Graham all but acknowledged that it could be months, if not years, before the shuttle programme - which was to have carried out 14 missions this year - will be back at work.

Even after the evidence has been assembled from the huge sea and air search yesterday, it could be a long time before the volumes of impounded material can be analysed and the causes of Challenger's end determined. There could be no flights of the shuttle until this task was completed and until the necessary modifications had been made to the three other shuttles in the fleet.

If the Apollo I disaster experience of 17 years ago provides a model, large-scale modifications will be required and even a total redesign of the shuttle could become necessary.

Eight Coast Guard and navy ships were involved in yesterday's search of the Atlantic, recovering debris up to 10 feet long from Challenger. 'We will search until darkness and then will evaluate what's been done,' said Commander Jim Simpson of the US Coast Guard. The search area has been extended into the ocean for about 60 miles, an indication of the full impact of the detonation for Challenger .

The debris collected is being impounded at Patrick air force base and will only be available to the board of inquiry. Some 10 US aircraft were involved in the hunt for evidence. But the real clues should be in the extensive computer and telemetry monitoring system maintained by Nasa.

However, there were some indications yesterday that this may not provide the clear statement of what happened which had been hoped for. According to Nasa sources quoted here, ' on first glance there was no anomalous data at all,' from the on-board IBM computers. In this area of investigation, attention is being focused on a strip of magnetised tape with thousands of instrument readings sent from the shuttle which, like an air craft's black box, ought to contain the answers.

Nevertheless, the absence of early warnings from the shuttle's processors and the complex web of sensors attached to them is a cause of some worry.